Recently, technology for thin and light display fields has accumulatively advanced and, thus, interest on materials for transparent electrodes is increasing. Materials for transparent electrodes must have electrical conductivity and transparent characteristics. Such transparent electrode materials are mainly used for high-tech display devices such as flat panel displays and touch screen panels.
Materials as transparent electrodes in such flat display fields have been generally used by coating a metal oxide electrode such as an indium tin oxide (ITO) electrode, an indium zinc oxide (IZO) electrode on a glass or plastic substrate through a deposition method such as sputtering. Although transparent electrode films manufactured using the metal oxides have high conductivity and transparency, but low frictional resistance and poor bendability. In addition, since natural reserves of indium, as a main material are limited, costs for indium are very high and indium has poor processability.
So as to overcome the processability problem described above, transparent electrodes using a conductive polymer such as polyaniline or polythiophene are being developed. Transparent electrode films using the conductive polymer may have high conductivity through doping, superior bondability of a coating film, and superior bendability. However, it is difficult for the transparent films using the conductive polymer to obtain superior electrical conductivity to the extent of being used for transparent electrodes. In addition, there is a problem that the transparent films using the conductive polymer have low transparency.
Therefore, carbon nanotubes have been developed as materials compared with indium tin oxides (ITO). Such carbon nanotubes are used in a variety of fields and, particularly, research into electrode materials is being actively performed due to superior electrical conductivity of the carbon nanotubes.
Graphite sheets of carbon nanotubes have a cylinder shape with nano-sized diameters and have a sp2 bond structure. Depending upon the angles and structures of the graphite sheets, the carbon nanotubes exhibit conductive or semiconductive characteristics. In addition, the carbon nanotubes are classified into single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), multi-walled carbon nanotubes (MWCNT), and rope carbon nanotubes, depending upon the number of bonds forming walls.
Especially, since the SWCNT has metallic characteristics and semiconductive characteristics, the SWCNT exhibits various electronical, chemical, physical and optical characteristics. Using such characteristics, more elaborate and integrated devices are realized. Examples of application fields of carbon nanotubes being currently studied include flexible and/or ordinary transparent electrodes, electrostatic dissipation films, field emission devices sheet type heating elements, optoelectronic devices, a variety of sensors, transistors, and the like.
Such carbon nanotubes are actively used as a conductive material but, when the carbon nanotubes are used in transparent electrodes, there is a problem that electrical conductivity is not sufficiently secured. However, since carbon nanotubes have relatively low haze values, transparency may be easily secured.
On the other hand, metal nanowires may be oxidized as time passes, and, when the metal nanowires are oxidized, electrical conductivity of transparent electrodes may be deteriorated, electrodes may be corroded and discoloration may be caused. Therefore, to use transparent electrodes for a long time, oxidation of metal nanowires has to be prevented. In addition, since metal nanowires exhibit superior electrical conductivity but decreased transparency, a technical solution to maintain electrical conductivity and secure transparency is required when the metal nanowires are applied.
International Patent Application Pub. NO. WO 2010/010838 discloses a transparent electrode comprising a transparent conductive layer composed of at least one conductive fiber type selected from carbon nanotubes and metal nanowires, and a surfactant. However, there is a problem that electrical conductivity and transparency are not superior due to poor dispersibility of the conductive fiber.
In order to address the aforementioned problems, the present inventors developed a transparent electrode having superior transmittance, electrical conductivity and transparency by applying a composition for a transparent electrode comprising (A) a carbon nanotube dispersing solution and (B) a metal nanowire solution having a zeta potential with the same polarity for superior dispersibility.